Ion source



`lune 7, 1955 Filed Aug. 28, 1952 FIELE INVENTORS 'ar/: /7Z ra/fz B Jaffa C ess June 7, 1955 M. G. INGHRAM ETAL ION SOURCE 2 Sheets-Sheet 2 Filed Aug. 2a,v 1952 MNM,

ww l w 2,7lllg354 N SOURCE Mark G. Inghram, Chicago, and David C. Hess, Riverdale, Ill'., assiguors to the United States of America as represented by the United States Atomic Energy @ommissi'on- Application August 28, 1952, Serial No. 306,844

3 Claims. (Cl. 2504.9)

The present invention relates to mass spectrometers,

and particularly beam sources of positive ions suitable for a use in mass spectronieters.

In recent years, the sensitivity of mass spectrometers has been greatly increased, largely as a result of the/use of electron multipliers to measure the charge oflions porated with beam sources yo'f ionsydepending upon sur- Y face ionization, since suchsources norinally produce small ion currents. However, [the present invention is also of advantage in reducingk the ion backgroundv peaks in electron bombardment beam ion sourcesvr or other beam sources when operated to produce small ion currents.

A more complete understanding of the present invention and its ramilications may be had from a reading of the present disclosure, particularly when viewed in the light of the drawings, in which:

Fig. 1 is a fragmentary view, partly in section, of a surface ionization beam source of positive ions constructed according to the teachings of the present invention;

Fig. 2 is a sectional View taken along line 2--2 of Fig. l

Fig. 3 is an elevational View, partly cut away in section, of an alternative embodiment of the beam ion source shown in Fig. 1;

Fig, 4 is a sectional view of a mass spectrometer incorporating the ion source shown in Fig. 1; and

Fig. 5 is a sectional View taken along line 5 5 of Fig. 3.

As shown in Fig. 4, the mass spectrometer is provided with a casing in which a vacuum is maintained. The casing 10 is generally Y-shaped and is provided with a magnet 12 at the junction of the two legs of the Y. Aniony source 14, which will be described in detail later, is disposed within one of the legs of theV casing 10 near one end thereof, and a collector 16 is disposed adjacent to the end of the other leg of the casing 10. Mass spectrometers of the type here generally described are well known in the art. Patent No. 2,551,544 of Alfred O. C. Nier and Mark G. Inghram, filed September 20, 1944, entitled Mass Spectrometer, discloses in detail a mass spectrometer of the type generally shown in Fig. 4.

The ion source 14, as illustrated in Figs. l, 2 and 4, has a supporting structure 20 with a pair of plates 22 and 24 aiiixed together by 4 posts 26 of electrically insulating material. One ot' the plates 22 is provided with a rectangular aperture 28 at its center, and the other plate 24 is provided with a collimating slit 30 at its center. A bracket 32 is attached to the plate 22, and

"\ yseparated and collected by the mass spectrometer(` vHow- 2,710,354 Fatented June 7, 1955 supports a lament 34 adjacent to the rectangular aperture 23 in the plate 22. The lilament 34l is. electrically insulated from the bracket 32 by insulators 36.

A slit plate 38 is mounted upon the posts 26 between the plates 22 and 24. The slit plate 38 is provided with a collimating slit 40 at its center, the collimating slit 40 being aligned with the collirnating slit 30 and the rectangular aperture 28. Electrically insulating sleeves 42 are disposed' about the posts 26 and space the slit plate 3S from the plate 24;

A grid plate 44 is mounted upon the posts 26 between the slit plate 38 and the plate 22, and the grid plate 44 is provided with a large rectangular aperture 46 at its center. The aperture 46 is covered with a' wire mesh grid 48 constructed of electrically conducting materials. The grid plate 44 is separated from the slit plate 38 by electrically insulating sleeves 50 disposed about the posts 26, and the grid plate 44 is also separatedl from the plate 22 by sleeves 52 of electrically insulating material dis'- posed about the posts 26.

Asshown in Fig. 4, a iirst accelerating voltage supply '54 is connected between the plate 22 and the slit plate l3 8 through a switch 56, and a second accelerating voltage supply 58 is connected between the plate 22 and the plate 24, through a switch 60. Both of these voltage supplies 54 and 58 impress a positive potential upon the plate* 22 relative to the slit plate 38- and the plate 24. A current supply 62 is connected to the iilament 34 through a switchl 64.

In order to place the mass spectrometer into operation, the filament switch 64 is closed heating the lament 34. The filament will liberate ions, as is well known in the art, and the ions will be of the materials with which the filament 34-has been constructed; Materials which make good filaments, such as tungsten, are often coated with other elements in order to form a filament which willI liberate the other more desirable ions. In this manner, the region immediately adjacent to the lament 34 becomes occupied with positive ions. However, since plate 22 is at a positive potential relative to slit plate 38 and plate 24', the ions thus liberated from the filament 34 are acceleratedv toward the slit plate 38. These ions readily pass through' the grid 48', since only a small percentageV of the area is obstructed by the grid 48, but only those ions directed toward the slit 4t) and the slit 30 will bel directed into the region of the magnetic eld produced by the magnet 12 or" the mass spectrometer. Many of the other ions which are not directed toward the slits 40 and 30 will strike the slit plates 38 or 24 andy be deposited thereon.

However, a number of negatively chargedY particles, such as negative ions and electrons, are liberatedfrom the slit plate 33 or plate 24 as a result of the bombardment of the plates 33 and 24 by the ions failing to pass through the slits 4d and 30. These particles are repelled.` from the slit plate 38 and plate 24` and attracted by the plate 22. Hence, these particles are accelerated in the direction of the lilament 34, and some bombard the lament 34, thereby producing tertiary positive ions. It l is believed that these tertiary ions produced by bombardment rather than by evaporation, as is the case of the positive ions originally produced by the filament 34, are or a different character than those produced by evaporation, and often account for the background levels observable in mass spectrometers.

However, a source of grid Voltage 66 is also connected between the plate 22 and the grid plate 44 through a switch 68, and places the grid 48 at a negative potential relative to the plate 22. As a result of the negative potential upon the grid 48, negative particles ejected from the slit plate 38 or the plate 24 are no longer accelerated toward the plate 22, and hence no longer bombard 3 the filament 34. In this manner, tertiary ions ejected from the filament 34 by bombardment are substantially reduced, and the background intensities in the mass spectrometer are also reduced.

It has been found that the potential applied to the grid plate 44 should be about one tenth as negative as the potential applied to the slit plate 38 and plate 24 relative to plate 22. In a particular construction, the plates 3S and 24 are about 3,000 volts negative with respect to plate 22, and the grid 48 is about 300 volts negative. In this same construction, the grid is spaced from plate 22 by 9 millimeters, and from slit plate 38 by 3 millimeters. Slit plate 38 is spaced from plate 24 by l2 millimeters.

Figures 3 and 5 show a second embodiment of a beam ion source constructed in accordance with the teachings of the present invention. This beam ion source has a supporting structure 20a with plates 22a and 24a and a mounting bracket 32a for a filament 34a, and is thus similar to the construction of the beam ion source shown in Fig. 1. A slit 36a is disposed centrally in the plate 24a. The plates 24a and 22a are separated by electrically insulating posts 26a which also support two pairs 7o and 78 of plates 80 and 82. The pairs of plates 76 and 78 are spaced between the plates 22a and 24a by spacers 42a, 50a and 52a. Each of the plates S0 and 32 are constructed with portions S4 normal to the rest ot the plates 80 and 32, the portion 84 of each plate Si? being disposed adjacent to and parallel to the portion 34 of a plate S2,

thereby providing gaps 86 and S8 between the plates 3) and 82 in both pairs of plates 76 and 7S.

As illustrated in Fig. 3, plates Si) are on opposite sides of the ion path, as are plates 82. A positive potential is applied to plates 82, and a negative potential is applied to plates Si), thus the positive plates S2 are on opposite sides of the ion path in the two pairs of plates 76 and 78. Also, plate 24a is placed at a negative potential relative to plate 22a, as in the first embodiment.

The positive ions liberated by the filament 34a will follow a trajectory generally indicated by the dashed line 9i), these positive ions being accelerated by the negative potential on plate 24a relative to plate 22a. The beam of positive ions is displaced in the direction of the negative plate 80 as it passes through the portion 84 of the first pair of plates 7S, and is returned by displacement in the opposite direction when passing through the portion 84 of the following pair of plates 76. However, some of the positive ions will strike the plate 24a adjacent to the slit 30a, thereby ejecting negative particles and electrons. The negative particles and electrons are accelerated toward the filament 34a, but due to their relatively low velocity and the differences in the potentials of plates 8i) and 82, the negative particles and electrons will follow a different trajectory than that of the positive ions, a possible trajectory being indicated by the dashed line 92. In this manner, the negative particles and electrons are blocked from bombarding the larnent 34a, and background ion peaks in the mass spectrometer as a result of tertiary ions liberated by electron bombardment of the filament 34a are eliminated.

It is thus clear to the man skilled in the art, that the mass spectrometers provided as a result of the present invention have lower background ion peaks than those previously known to the art, and that the beam sources of ions here disclosed produce relatively few ions of undesired mass numbers. Many other modifications and advantages of the present invention will readily appear to the man skilled in the art, and hence it is intended that the scope of the present invention be limited only by the appended claims and not by the foregoing disclosure.

Vfhat is claimed is:

l. A beam source of positive ions comprising, in cornbination, an evacuated housing, a source of positive ions, a plate provided with a collimating slit disposed within the housing, means to maintain a potential gradient becoming more negative between the source and the slit, and an electrostatic detiection negative particle trap disposed between the ion source and the slit, said trap transmittin g the beam of positive ions from the source through the collimating slit in a substantially straight path and trapping negative particles traveling toward the ion source.

2. A beam source of ions comprising, in combination, an evacuated housing, a source of positive ions disposed within the housing, a plate having a collimating slit disposed within the housing and spaced from the source of ions, means to maintain a potential gradient becoming increasingly negative from the source to the slit, a first pair of deflection plates mounted between the source and the slit, said plates being electrically insulated frm each other and disposed upon opposite sides l the path between t'ne source and the slit, a second'pair of deflection plates disposed between the first pair of deflection plates and the plate,` said plates being parallel to the first pair of plates, means taapplya potential gradient between the rst pair of plates, and means to apply a potential gradient between the second pair of plates, said potential gradient being opposite to that applied across the first pair of plates.

3. A beam source of positive ions comprising, in combination, an evacuated housing, a surface evaporation ion source disposed within the housing, including a filament, a plate having a collimating slit disposed within the housing and spaced from the source, means to maintain an ever increasing negative potential gradient between the source and the slit, a first pair of defiection electrodes mounted between the source and the slit, said electrodes being disposed about opposite sides of the path between the source and the slit and electrically insulated from each other, a second set of defiection electrodes disposed be tween the first set of electrodes and the slit, said second set of electrodes being parallel to the first and electrically insulated from each other, means to apply a potential across the first pair of electrodes, and means to apply a potential across the second pair of electrodes, the electrodes in each pair on the same side of the path between the filament and the slit being of equal but opposite potcntial.

References Cited inthe file of this patent UNITED STATES PATENTS 2,583,121 Reynolds Jan. 22, 1951 2,587,575 Washburn Feb. 26, 1952 2,623,l79 Neuland Dec. 23, 1952 

1. A BEAM SOURCE OF POSITIVE IONS COMPRISING, IN COMBINATION, AN EVACUATED HOUSING, A SOURCE OF POSITIVE IONS, A PLATE PROVIDED WITH A COLLIMATING SLIT DISPOSED WITHIN THE HOUSING, MEANS TO MAINTAIN A POTENTIAL GRADIENT BECOMING MORE NEGATIVE BETWEEN THE SOURCE AND THE SLIT, AND AN ELECTROSTATIC DEFLECTION NEGATIVE PARTICLE TRAP TRANSPOSED BETWEEN THE ION SOURCE AND THE SLIT, SAID TRAP TRANSMITTING THE BEAM OF POSITIVE IONS FROM THE SOURCE THROUGH 